In hydraulic working support systems, such as are used in underground mining, non-return check valves are associated with the hydraulic props and the additional hydraulic appliances, such as for example the shifting rams and walking mechanisms, the alignment devices and roof bar adjusting devices, in order to isolate the pressure chambers of the hydraulic appliances. Known non-return valves are opened by means of control pistons, which receive hydraulic pressure control signals, in order, for example to retract the props, or otherwise to reduce the pressure in the working chambers or, to supply the working chambers with the hydraulic high pressure fluid, for example for extending and setting the props. Non-return valves of the construction which is known from GB 2216 238 (which corresponds to U.S. Pat. No. application 312 075 filed Feb. 16, 1989 and assigned to the same assignee as this application) have proved successful for the above mentioned purposes. As the closure member, these known non-return valves each use a valve sleeve guided on a rod. The valve sleeve is subjected to pressure-equalization by means of a compensating piston on the rod and is held on a valve seat substantially solely by the force of a relatively weak auxiliary spring. In the closing state of the valve, a main valve spring keeps the rod in abutment with a housing shoulder. Due to this the operational reliability of the valve is increased and at the time of the closing operation, the closure member is prevented from striking too hard against the valve seat, which may lead to damage and to failure of the valve.
One drawback of the non-return valves, which are opened hydraulically and are known in various constructions, consists above all in the case of failure of the hydraulic control, for example in the case of breakdown of the pump pressure instanced by the fracture of a cable or in the case of fracture of a valve component. In these circumstances, the non-return valve can no longer be opened and accordingly the pressure in the pressure chamber shut-off by the latter cannot be relieved without dismantling the valve and corresponding danger due to external intervention. Especially if the hydraulic appliances are the props of a support, this may lead to considerable disturbances in the operating sequence and also to considerable danger for the face workers, since the props or other hydraulic appliances are blocked and unable to extend or retract. Although it is well known to use additional pressure relief valves to protect the equipment these valves open automatically when a pre-determined excess pressure above normal pressure is exceeded and do not assist in the problem identified above.
It is an object of the invention to provide apparatus which will combat the aforementioned difficulties and problems, without excessive expenditure.